Discriminating apparatus for moving translucent sheets

ABSTRACT

Apparatus for discriminating between moving translucent sheets comprising a sheet transportation means adapted to convey the sheets in spaced relationship along a flow-line, a photoelectric detection device sited on one side of the flow-line, a first differential amplifier having an input in circuit with the said detection device and adapted to produce a first output signal every time the photoelectric device detects attenuation of the light source in excess of a first predetermined value, and a second differential amplifier having an input also in circuit with the said detection device and adapted to produce a second output signal every time the photelectric device detects attenuation of the light source in excess of a second different predetermined value.

United States Patent 1191 Pettet 145] Reissued Dec. 23, 1975 1DISCRIMINATING APPARATUS FOR 3,520,404 7/1970 Pine 209/1117 MOVINGTRANSLUCENT SHEETS 3,525,433 8/1970 Babb 209/1 1 117 3,578,315 5/1971Milford 271/263 Inventor! Ronald Sidney Pettel, Portsmouth, 3,611,88710/1971 Shibanuma 93/93 c England [73] Assignee: De La Rue InstrumentsPrimary Examiner Ev0n C Bhmk [22] Filed: Sept. 17, 1973 [21] Appl. No.:397,622

Related US. Patent Documents Reissue of:

[64] Patent No.: 3,731,916

Issued: May 8, 1973 Appl. No; 148,744 Filed: June 1, 1971 [30] ForeignApplication Priority Data June 4, 1970 United Kingdom 26,940/70 [52] US.Cl. 271/263; 250/559; 250/560; 271/64 [51] Int. Cl. B6511 7/14 [58]Field of Search 271/263, 262, 258, 64; 209/1l1.7; 250/559, 560, 360;93/93 C [56] References Cited UNITED STATES PATENTS 3,186,708 6/1965Hinz 271/263 Limited, London, England Assistant ExaminerBruce H. Stoner,Jr. Attorney, Agent, or FirmBaldwin, Wight & Brown 57 ABSTRACT Apparatusfor discriminating between moving translucent sheets comprising a sheettransportation means adapted to convey the sheets in spaced relationshipalong a flow-line, a photoelectric detection device sited on one side ofthe flow-line, a first differential amplifier having an input in circuitwith the said de tection device and adapted to produce a first outputsignal every time the photoelectric device detects attenuation of thelight source in excess of a first predetermined value, and a seconddifferential amplifier having an input also in circuit with the saiddetection device and adapted to produce a second output signal everytime the photelectric device detects attenuation of the light source inexcess of a second different predetermined value.

17 Claims, 5 Drawing Figures Reissued Dec. 23, 1975 Sheet 1 of 3 R6.28,660

Reissued Dec. 23, 1975 Sheet 2 of3 Re. 28,660

OUTPUT F02 A L m/c/eussszs.

OUTPUT F0 M096 7764 ONE THICKNESS.

PULSE COUN TER GATE T T ER N NSN W was 7 a w Np U5 IN m W m r M n m C Hs w W ow 7 Reissued Dec. 23, 1975 shw 3 Ora Re. 28,660

INHIBIT II G. 4-

DISCRIMINATING APPARATUS FOR MOVING TRANSLUCENT SHEETS Matter enclosedin heavy brackets I: 1 appears in the original patent but forms no partof this reissue specification; matter printed in italics indicates theadditions made by reissue.

This invention relates to apparatus adapted to dis criminate betweenlengths and thickness of translucent sheet material travelling along aflow-line at high speed and optionally to count some or all of suchsheets. More especially, the invention relates to such apparatus whichincludes a photoelectric detection means and an associated light sourcedisposed on opposite sides of the flow-line in such a manner that thephotoelectric means is subjected to variations of light intensity duringthe passage of the sheets.

BACKGROUND OF THE INVENTION In many sheet feeding machines, for example,in the case of machines adapted to feed and count treasury notes, it isessential that an exact count is recorded of the number of piecesactually delivered. To this end it is usual to provide means whichensure that multiple or superposed notes are not counted as a singleunit.

A known method of achieving the above requirement is to feed the notesthrough a critical gauging throat which may be arranged to hold backsuperposed notes, or to stop the machine when sheets of abnormalthickness are present, or more preferably to actuate a divertor meanswhich serves to remove multiple notes from the flow-line before they arefed to a counting and delivery stage. An example of the latterarrangement is described in our British Pat. No. 898,7l5 in which thegauging throat comprises a nip of a critical dimension formed between apair of driven gauging rollers. It is so arranged that normal thicknessnotes pass through the nip, but multiple or excess thickness-notes causea displacement of the axis center of one of the rollers which bringsinto effect a solenoid actuated divertor. Counting is carried out bymeans of an electrical makeand-break circuit disposed downstream of thedivertor which advances a digital counter in an incremental manner. Suchan arrangement possesses a finite operating speed because of themechanical inertia of the detection means and, furthermore, because ofthe difficulties in differentiating between relatively small differencesof thickness at high speed, considerable constructional problems arecreated. It has been found in practice that the gauging nip must beadjusted within fine limits to promote the desired sensitivity toabnormal thickness and furthermore mechanical wear of certain criticalcomponents necessitates careful maintenance to retain the desired degreeof reliability.

SUMMARY OF THE INVENTION It is one object of the present invention toprovide apparatus for discriminating between sheets of theabove-described kinds without the need for a precise mechanical gaugingthroat.

According to the present invention apparatus for discriminating betweenmoving translucent sheets comprise a sheet transportation means adaptedto convey the sheets in spaced relationship along a flow-line, aphotoelectric detection device sited on one side of the flow-line and anassociated light source sited on the other side of the flow-line, afirst differential amplifier having an input in circuit with the saiddetection device and adapted to produce a first output signal every timethe photoelectric device detects attenuation of the light source inexcess of a first predetermined value, and a second differentialamplifier having an input also in circuit with the said detection deviceand adapted to produce a second output signal every time thephotoelectric device detects attenuation of the light source in excessof a second different predetermined value. Thus in one embodiment of theinvention apparatus for handling sheets of a like-kind may be so adaptedthat the output of the said first differential amplifier controls afirst digital counting circuit to record the total quantity of singlesheets or groups of adhered-together" sheets fed along the flow-line,and that the output of the said second differential amplifier controls asecond digital counting circuit to record solely the number of groups ofadhered-together" sheets.

Preferably the apparatus also includes a sheet divertor means disposeddownstream of the said detection device and under the control of thesaid second differential amplifier, the arrangement being such that thedivertor, which is normally inoperative, is moved to an operativeposition every time the said second output signal is created. In aspecific embodiment it may be arranged for a count pulse to be inhibitedfrom the said first counting circuit every time the divertor is moved tothe operative position thereby to record only the passage of singlesheets.

Preferably the apparatus also includes a pulse-counter unit fed from apulse generator via a gate under the control of the output of the firstdifferential amplifier, said unit being capable of ascertaining thelength of a sheet or the overall length of overlapping sheets in thedirection of the flow-line thereby, in conjunction with the output ofthe second differential amplifier, to detect the presence of superposed,partially overlapping or edge-abutting sheets.

Preferably the said pulse generator is driven in unison with the lineartravel of the sheets in the flowline. In a convenient form the pulsegenerator comprises a circumferentially perforated or notched wheelmember positively driven by the drive mechanism of the said sheettransportation means, and adapted to influence a transducer which may beof the optical or magnetic kind.

One embodiment of the invention will now be described with reference tothe accompanying drawings of which:

FIG. I is a side elevation of a banknote feeding and transportationmechanism showing the basic parts thereof;

FIG. 1a is a fragmentary end view of some of the parts of the mechanismof FIG. 1;

FIG. 2 is a diagram of part of an electrical circuit for performing theoptically discriminating aspect of the invention; I: and 1 FIG. 3 is aschematic block plan showing the interconnection of various electronicassemblies useful in the performance of the invention and FIG. 4 isanother block plan showing the interconnection ofdigital countersoperative by the electrical circuit of FIG. 2.

In FIG. 1, the banknote feeding, transporting and deflecting mechanismsare substantially similar to those disclosed in our British Pat. No.893,715 and comprise essentially an inclined stack support member 1adapted to support a stack of banknotes N on edge. The notes are biassed(by means not shown) towards an oscilla- Lory suction arm 2 providedwith an angled suction port 3 which contacts the lower central face ofthe frontnost note in the stack and feeds it into the nip ofa pair 3fprimary feed rollers 4 and 5. The lower roller 5 is )rovided with acut-away portion 6 to facilitate entry of :he leading side edge of thenote. The arm is oscillated )y means of a tubular shaft member 7 whichis adapted .o be moved in a bi-rotational manner by a cam system irivenin unison with the primary feed rollers 4 and 5;

.he interior of the shaft 7 is in communication with a lOUICC of suction(not shown). When the circumferen- .ial surfaces of the feed rollersco-act, the note is pulled rom the face of the suction port 3, and ispassed be- .ween a light L1 and an associated photoelectric de- .ectorPET, and thence into the nips of three pairs of :onveying rollers 8,9;10,1]; and 12,13 respectively. The nips of the primary and conveyingrollers collecively define a substantially linear flow-line for theiotes which terminates at the arrow 14; delivery means )eyond the arrowform no part of the present inven- .ion.

A note divertor means provided between the conveyng rollers 10,11 andl2, 13 comprises a plurality of spaced fork members [5 rigidly attachedto a shaft 16 vhich also includes an actuating lever 17 connected to .heslidable armature 18 of a solenoid l9 by means of 1 connecting link 20.The divertor assembly is biassed .0 an inoperative position, as shown bymeans of a .ension spring 21. An auxiliary rubber-surfaced driven 'oller22 is provided above the upper conveying roller l0, and a *reject notehopper 23 is provided above be rollers 4 and 8 to receive divertednotes. Energizavion of solenoid 19 causes the divertor assembly to noveto the position indicated by the chain-dotted line, whereupon the tipsof the fork members descend nto the flow-line thereby to cause a note ornotes to be Jropelled around 180 of the circumference of the 'oller l0and thence into the hopper 23.

To facilitate the passage of notes through the roller iystem a series ofparallel guide members, one of which s indicated by symbol 24, areprovided below the flowme.

The entire roller system is driven at a constant peipheral speed througha gear train generally indicated it 25, by an electric motor 26 via atwo-stage gear 'eduction disposed on secondary shafts 27 and 28. Theihaft 27 (see also FIG. la) mounts a fixedly attached :lisc member 29which is provided with an equally )itched circumferential row of holes30. A light source L2 and an assocated photoelectric detector PEZ arejisposed in alignment with the holes 30 on the opposite ;ides of thedisc.

The electronic circuitry of the apparatus will now be iescribed withreference to FIGS. 2 and 3. it should be ioted that the two outputs xand y of HG. 2 are coniected respectively to the left-hand side of FIG.3.

The output of the photoelectric detector PE! is fed nto a linearamplifier the output of which feeds into :wo substantially similarnetworks, and thence through 'espective differential amplifiers topoints x and y. The assembly 1 serves to provide an output signal at xfor :ach single note or group of partially or wholly supernosed notesthat pass between L] and PEI, and the assembly ll (which is of coursepre-set in a different manner), serves to provide an output signal at yfor 4 each multiple thickness note or notes, or each single note havingan abnormally high opacity that passes between L1 and PEl.

The output of the linear amplifier 50 passes through a rectifier MRwhich, with a capacitor C1, forms a peak level detection means. When nonote is interposed between L1 and PE] the linear amplifier produces apeak output proportional to the intensity of the light falling on thephotoelectric detector PB]. The rectifier MR permits only the positivepeak to charge the capacitor Cl and effectively prevents reverse flow ofcurrent from C1 when the light falling upon PE] is attenuated by thepresence of a note.

An R/C network comprising C1, R1, RVl and R2 provides a time-constantsufficiently long to prevent any significant discharge ofCl during theperiod when light is attenuated. The arrangement is self-compensatinginsofar that a gradual fall-off of light output created by the ageingeffect of the light source, a deterioration in the sensitivity of thephotoelectric detector or a reduction of gain of the linear amplifier,is compensated for automatically because the charge of Cl will alsoreduce at the same rate.

Having charged the peak level detection means up to the voltage producedby the unattenuated light source, a proportion of it is coupled to theinverting input of the differential amplifier via the slider of thepotentiometer RV 1. The potentiometer thus provides a presetting meansfor adjusting the sensitivity of the assembly. The full output of thelinear amplifier, taken from a point upstream of the rectifier MR, isconnected to the positive non-inverting" input of the differentialamplifier. Hence, when the light falling upon the photoelectric detectoris unattenuated the voltage to the non inverting" input providespositive feed-back so that the output it of the differential amplifierwill be a maximum positive. Conversely. when the light level isattenuated, the output of the linear amplifier will fall to below thelevel set on the inverting" input with the result that the differentialinput will be "inverted" thereby to produce a negative output it fromthe differential amplifier. A positive feed-back loop comprising theresistor R4 ensures that the output x will be a maximum negative.

The assembly II, which terminates at an output y, is substantiallysimilar to that described above and serves to produce an output pulse ofchanged polarity when the light is attenuated to a greater degree, forexample on the passage of a plurality of adhered-together notes.

The means for discriminating between the length of a single sheet andthe overall length of superposed sheets, as measured in the direction ofthe flow-line. will now be described.

Operation of the roller system of the machine also causes, through themedium of the gearing system, the perforated disc member 29 to rotate,with the result that a succession of electrical pulses is created in thephotoelectric detector PE2. These pulses are fed into a Pulse Countervia a Latched Gate" (see FIG. 3) which is opened at the commencement ofevery period of attenuation of PEI, via the electronic assembly 1 andclosed after a preset number of pulses have been fed into the PulseCounter". The number of pulses fed into the Pulse Counter" is directlyproportional to the angular displacement of the shafts in the drivemechanism, and hence to the length of the sheet being fed.

The Pulse Counter" provides outputs in accordance with three differentpreset and ascending numbers of datum counts which permit decisions tobe effected in accordance with the overall length of a sheet or sheetspassing through the optical system. These counts are routed in thefollowing manner:

COUNT A into a Divert Command Generator" which also receives signalsfrom the electronic assembly II via connection y. The Divert CommandGenerator" is provided with two alternative outputs viz, a YES output tothe divertor solenoid l9 and a Count Pulse Generator", or a NO output toa Warning Generator".

If signals from connections y are received before COUNT A is reached,the YES output is obtained, the divertor solenoid 19 is actuated and anINHIBIT signal is fed to the COUNT PULSE GENERATOR. If COUNT A isexceeded before signals are received from the connection y, the NO-output is energized and the WARNING GENERATOR is energized.

COUNT B into the Warning Generator" (which also receives signals fromthe x connection during the entire period of the attenuation of thelight of LI) and thence to a warning device shown diagrammatically atsymbol SI. COUNT C into the Count Pulse Generator (which, as mentionedabove, also receives INHIBIT signals when the divertor is actuated) andthence into a digital counter and display unit shown diagrammatically atsymbol 52. In addition a RE-SET pulse is fed back to unlatch the LATCHEDGATE upon attainment of COUNT C. Actuation of the Apparatus, includingexamples of correct and incorrect note feeding.

Energization of the driving motor 26 will cause banknotes to be fedsequentially from the front of the stack N into the roller system alongthe flow-line thereby to attenuate in an intermittent manner the lightfalling upon the photoelectric detector PEI. Pulses will also begenerated by the photoelectric detector PE2 through rotation of the discmember 29.

Case I The passage of a normal length single note will record a countingpulse in the counter unit 52 in the following manner:

When the leading edge of the note attenuates the light source a signalfrom the all-thickness" assembly I opens the Gate which allows pulsesfrom PE2 to pass into the Pulse Counter. For a note of normal length,the light attenuation signal from x into the WARN- ING GENERATOR"terminates before COUNT B is reached and thus no warning is generated.When the Pulse Counter" reaches COUNT C" a count pulse is generated anda count of one is registered in the counter unit 52.

Case 2 The passage of two superposed or substantially superposed noteswill be rejected in the following manner:

In a manner similar to that described in (1) above, the Gate passespulses to the Pulse Counter" but in this case, a multiple-thicknesspulse is generated by the assembly II through the connection y andDivert Command Generator. If the y signal, which is indicative of thecommencement of the multiple thickness during the passage of the sheets,is created before COUNT A is reached the sheets are capable of diversionand accordingly the Divert Command Generator generates a YES signal andenergizes the solenoid 19 of the divertor thereby to cause thesuperposed notes to be com veycd into the reject note hopper 23. At thesame time the count pulse at COUNT C" is inhibited and no count isrecorded in the counter unit 52. If signal y is created after COUNT A isreached, diversion is not possible and the superposed or substantiallysuperposed sheets are treated as partially overlapping sheets (see Case3 below).

Case 3 The passage of two partially-overlapping notes will be handled inthe following manner:

The cycle of operation is initiated as in Case 2 above but in this caseCOUNT A is reached before the creation of the y signal and the DivertCommand Generator does not energize the solenoid 19 of the divertor.This prevents the mechanism from attempting to divert a note, theleading edge of which has already passed below the tips of the divertorfork members 15. Instead, a NO signal is applied to the Warning Generator" which give a warning signal and the count pulse is not inhibited.

Case 4 The passage of two edge-abutting notes (viz. with no overlap) ishandled as follows:

The cycle of operation is initiated as in Case 3 above, but in this caseno multiple-thickness is detected. However, when COUNT B is reached theoutput of the all-thickness assembly I at connection x into the WARNINGGENERATOR is still maintained thus indicating the presence of anabnormally long article. The Warning Generator" initiates a warning andthe Count Pulse Generator is activated as in Case 3.

It will be appreciated that the above described apparatus is capable ofcounting and discriminating translucent sheets at a very high speed, theupper limit of which is dependent only upon the mechanical limitationsof the feeding mechanism. Furthermore, the photoelectric detection meansand associated circuitry is self-compensating and thus after an initialsetting-up, is capable of operating for long periods without attention.

It will further be appreciated that the complete elimination of the needfor a critical gauging throat to detect multiple-thicknesses hassimplified the mechanical construction of machines of this general typeand avoided the need for frequent adjustment and maintenance.

Reference is now made to FIG. 4 wherein a counter circuitry is shownincluding afirst digital counter 53 and a second digital counter 54. ysignals pass into the second digitalcounter 54 to advance the same oneincrement for each production ofa y signal. x signals pass through thefirst digital counter 53 to normally advance the same one increment foreach production of an x signal. However, the circuitry includes meansfor producing an IN- HIBIT signal upon each generation of a y signal toprevent the first digital counter 53 for advancing an increment whenboth x and y signals are generated.

What we claim is:

1. Apparatus for discriminating between moving translucent sheetscomprising a sheet transportation means for conveying the sheets inspaced relationship along a flow-line, a single photoelectric detectiondevice positioned on one side of the flow-line and an associated lightsource positioned on the other side of the flow-line, a firstdifferential amplifier means for producing a first output signal everytime the photoclectric detection device detects attenuation of the lightsource in excess of a first predetermined value and iaving an input incircuit with the said detection device. and a second differcntialamplifier means for reducing a second output signal every time thephoto- :lcctric detection device detects attenuation of the ight sourcein excess of a second different predeter' nincd value greater than saidfirst predetermined value ind having an input also in circuit with thesaid detecion device and means for receiving and utilizing said iutputsignals to discriminate between sheets which proiuce solely saidfirstoutput signal and sheets which proluce both said first and second outputsignals.

2. Apparatus as claimed in claim I in which the first )utput signalserves to indicate the passage of (a) each .ingle sheet having anopacity greater than the first redetermined value and less than thesecond predeter nined value; (h) each folded single such sheet; (c) :achsingle sheet having an opacity greater than the second predeterminedvalue; and ((1) each group of )artially or wholly superposed singlesheets having an pacity greater than the second predetermined value. indthe second output signal serves to indicate the )assage of any sheet incategory (b), (c) or (d).

I: 3. Apparatus as claimed in claim I together with neans for couplingthe first output signal to a first ligital counter for actuating thesame and means for :oupling the second output signal to a second digital:ounter for actuating the same] [4. Apparatus as claimed in claim 3together with neans for inhibiting the first output signal to the firstiigital counter upon the production of the second out- Jut signal. I

5. Apparatus as claimed in claim 1 in which said neans for receiving andutilizing said output signals ncludes a sheet divertor [is provided inthe flowine at a point downstream of the photoelectric detec- .iondevice, said divertor being normally inoperative with respect to thesheets, but being operable to move nto the flow-line every time the saidsecond output iignal is produced.

6. Apparatus as claimed in claim 5 in which said neans for receiving andutilizing said output signals additionally] includes lengthdiscrimination means "or inhibiting actuation of the sheet divertor ifthe ength of a sheet, or the overall length of a group of Jartially orwholly superposed sheets in the direction along the flow-line, exceeds apredetermined dimension.

7. Apparatus as claimed in claim [6 in I which I: the said said meansfor receiving and utilizing said )utput signals includes a digitalcounter and length discrimination discriminator means [comprises 1'ncluding [a pulse generator driven in unison with the sheettransportation 1 pulse-generating means, a pulse- :ounting means [,1 andgate-switching means [for directing arranged to be opened by said firstoutput signal and to direct a train of pulses from the pulse gen eratorto the pulse-counting means whilst a sheet is present between the lightsource and the photoelectric detection device, and a count-discriminatorfor activating the sheet divertor only if less than a predeterminednumber of pulses are directed into]; the pulse-count ing means beingoperable to produce successively a first sheeelength-indicative datumcount, (1 second sheetlength-indicative datum count and a digital countpulse for advancing said digital counter by one increment.

8. Apparatus as claimed in claim 7 in which the said pulse generatorcomprises a circumferentially perforated wheel positively driven by thesheet transportation means, a second light source disposed on one sideof the wheel in alignment with the said perforations, a secondphotoelectric-detection means disposed on the other side ofthe wheel inalignment with the said perforations and operable upon rotation ofthcwheel to feed a train of pulses into the said gateswitching means.

9. Apparatus as claimed in claim [5] 7 so constructed that the [creationtermination of said first output signal and the termination thereof]before the attainment of the second datum count [and the creation of thesaid second output signal before the attainment of the first datumcount] indicates the passage of one or more sheets having an over-alllength within a predetermined range of lengths plurality of overlappingsheets and further adapted to cause said divertor to move into theflow-line upon the passage of such sheets].

10. Apparatus as claimed in claim [5] 7 so can structed that the[creation termination of [the said first output signal [and thetermination thereof 1 after the attainment of the second datum count[and the creation ofthe second output signal after the attainment of thefirst datum count indicates the passage of I: a plurality of overlappingsheets, and further adapted to inhibit the movement of said divertorinto the flowline and to actuate a warning means one or more sheetshaving an overall length in excess ofa predetermined length.

ll. Apparatus as claimed in claim 1 in which each differential amplifiermeans is provided with manually adjustable means adapted to permit theinitial setting up of its output level.

12. Apparatus as claimed in claim 11 in which said input to saiddifferential amplifier means includes one input dependent upon theeffect of attenuated light upon said photo-electric device and a secondinput dependent upon the effect of direct light upon said photoelectricdevice, said second input including said adjustable means.

I: 13. Apparatus as claimed in claim I which additionally includeslength discriminator means including a pulse generator driven in unisonwith the sheet transportation means, a pulse-counting means, gateswitching means for directing a train of pulses to the pulsecountingmeans during production of said first output signal, the pulse counterbeing operable to produce successive first and second sheetlength-indicative datum counts] 14. Apparatus as claimed in claim 13 soconstructed that the creation of the first output signal and thetermination thereof after the attainment of the first datum count. butbefore the attainment of the second datum count, indicates the passageof a sheet of predetermined opacity and length] [15. Apparatus asclaimed in claim 13 so constructed that the creation of the first outputsignal and the termination thereof after the attainment of the seconddatum count indicates the passage of at least one sheet having anoverall length in excess of a predetermined length. I

[16. Apparatus as claimed in claim 15 including warning means operableupon the passage of one or more sheets having an overall length inexcess of a predetermined length. I

[17. Apparatus as claimed in claim 13 in which the said pulse generatorcomprises a circumferentially perforated wheel positively driven by thesheet transportation means. a light source disposed on one side of thewheel in alignment with the said perforations, a photoelectric detectionmeans disposed on the other side of the wheel in alignment with the saidperforations and adapted upon rotation of the wheel to feed a train ofpulses into the said gate-switching means] 18. Apparatus as claimed inclaim 9 which additionally includes a sheet divertor in the flow-linedownstream of the photo-electric detection device, said divertor beingnormally inoperative with respect to the sheets but being available tomove into theflow-line everytime the second output signal is produced,and a count-pulse inhibit switch means operable in unison with everysaid divertor movement to prevent said digital count pulse from advancing said digital counter by one increment.

19. Apparatus as claimed in claim 10 additionally including a warningmeans operable on the passage of one or more sheets having an overalllength in excess of the predetermined length.

20. Apparatus as claimed in claim 19 additionally including a sheetdivertor in the flow-line downstream of the photoelectric detectiondevice, said divertor being normally inoperative with respect to thesheets but being operable to move into the flow-line upon the productionof the second output signal as a consequence of the passage ofa sheet orsheets having an overall length within a predetermined range of lengthsLII 21. Apparatus as claimed in claim 20 additionally including acount-pulse inhibit means operable in unison with every said divertormovement to prevent said digital count pulse from advancing said digitalcounter by one increment.

22. Apparatus as claimed in claim 2] additionally including meansoperable to maintain said divertor in its normal inoperative positionupon the production of the second output signal as a consequence of thepassage of one or more sheets having an overall length greater than anylength within a predetermined range of lengths.

23. Apparatus as claimed in claim I wherein said means for receiving andutilizing said output signals includes a first digital counter coupledto said first differential amplifier means for receiving said firstoutput signal therefrom and to be actuated thereby, and a second digitalcounter coupled to said second differential amplifier means forreceiving said second output signal therefrom and to be actuatedthereby.

24. Apparatus as claimed in claim 23 wherein saidfirst digital counteris coupled to said second differential amplifier means for inhibiting afirst output signal to said first digital counter upon the production ofa second output signal.

1. Apparatus for discriminating between moving translucent sheetscomprising a sheet transportation means for conveying the sheets inspaced relationship along a flow-line, a single photoelectric detectiondevice positioned on one side of the flow-line and an associated lightsource positioned on the other side of the flow-line, a firstdifferential amplifier means for producing a first output signal everytime the photoelectric detection device detects attenuation of the lightsource in excess of a first predetermined value and having an input incircuit with the said detection device, and a second differentialamplifier means for producing a second output signal every time thephotoelectric detection device detects attenuation of the light sourcein excess of a second different predetermined value greater than saidfirst predetermined value and having an input also in circuit with thesaid detection device and means for receiving and utilizing said outputsignals to discriminate between sheets which produce solely said firstoutput signal and sheets which produce both said first and second outputsignals .
 2. Apparatus as claimed in claim 1 in which the first outputsignal serves to indicate the passage of (a) each single sheet having anopacity greater than the first predetermined value and less than thesecond predetermined value; (b) each folded single such sheet; (c) eachsingle sheet having an opacity greater than the second predeterminedvalue; and (d) each group of partially or wholly superposed singlesheets having an opacity greater than the second predetermined value,and the second output signal serves to indicate the passage of any sheetin category (b), (c) or (d).
 5. Apparatus as claimed in claim 1 in whichsaid means for receiving and utilizing said output signals includes asheet divertor (is) provided in the flow-line at a point downstream ofthe photoelectric detection device, said divertor being normallyinoperative with respect to the sheets, but being operable to move intothe flow-line every time the said second output signal is produced. 6.Apparatus as claimed in claim 5 in which said means for receiving andutilizing said output signals (additionally) includes lengthdiscrimination means for inhibiting actuation of the sheet divertor ifthe length of a sheet, or the overall length of a group of partially orwholly superposed sheets in the direction along the flow-line, exceeds apredetermined dimension.
 7. Apparatus as claimed in claim (6 in) 1 which(the said) said means for receiving and utilizing said output signalsincludes a digital counter and length (discrimination) discriminatormeans (comprises) including (a pulse generator driven in unison with thesheet transportation) pulse-generating means, a pulse-counting means (,)and gate-switching means (for directing) arranged to be opened by saidfirst output signal and to direct a train of pulses from the pulsegenerator to the pulse-counting means (whilst a sheet is present betweenthe light source and the photoelectric detection device, and acount-discriminator for activating the sheet divertor only if less thana predetermined number of pulses are directed into) ; the pulse-countingmeans being operable to produce successively a firstsheet-length-indicative datum count, a second sheet-length-indicativedatum count and a digital count pulse for advancing said digital counterby one increment .
 8. Apparatus as claimed in claim 7 in which the saidpulse generator comprises a circumferentially perforated wheelpositively driven by the sheet transportation means, a second lightsource disposed on one side of the wheel in alignment with the saidperforations, a second photoelectric-detection means disposed on theother side of the wheel in alignment with the said perforations andoperable upon rotation of the wheel to feed a train of pulses into thesaid gate-switching means.
 9. Apparatus as claimed in claim (5) 7 soconstructed that the (creation) termination of said first output signal(, and the termination thereof) before the attainment of the seconddatum count (and the creation of the said second output signal beforethe attainment of the first datum count) indicates the passage of one ormore sheets having an over-all length within a predetermined range oflengths (plurality of overlapping sheets and further adapted to causesaid divertor to move into the flow-line upon the passage of suchsheets) .
 10. Apparatus as claimed in claim (5) 7 so constructed thatthe (creation) termination of (the) said first output signal (and thetermination thereof) after the attainment of the second datum count (andthe creation of the second output signal after the attainment of thefirst datum count) indicates the passage of (a plurality of overlappingsheets, and further adapted to inhibit the movement of said divertorinto the flow-line and to actuate a warning means) one or more sheetshaving an overall length in excess of a predetermined length. 11.Apparatus as claimed in claim 1 in which each differential amplifiermeans is provided with manually adjustable means adapted to permit theinitial setting up of its output level.
 12. Apparatus as claimed inclaim 11 in which said input to said differential amplifier meansincludes one input dependent upon the effect of attenuated light uponsaid photo-electric device and a second input dependenT upon the effectof direct light upon said photoelectric device, said second inputincluding said adjustable means.
 18. Apparatus as claimed in claim 9which additionally includes a sheet divertor in the flow-line downstreamof the photo-electric detection device, said divertor being normallyinoperative with respect to the sheets but being available to move intothe flow-line everytime the second output signal is produced, and acount-pulse inhibit switch means operable in unison with every saiddivertor movement to prevent said digital count pulse from advancingsaid digital counter by one increment.
 19. Apparatus as claimed in claim10 additionally including a warning means operable on the passage of oneor more sheets having an overall length in excess of the predeterminedlength.
 20. Apparatus as claimed in claim 19 additionally including asheet divertor in the flow-line downstream of the photoelectricdetection device, said divertor being normally inoperative with respectto the sheets but being operable to move into the flow-line upon theproduction of the second output signal as a consequence of the passageof a sheet or sheets having an overall length within a predeterminedrange of lengths
 21. Apparatus as claimed in claim 20 additionallyincluding a count-pulse inhibit means operable in unison with every saiddivertor movement to prevent said digital count pulse from advancingsaid digital counter by one increment.
 22. Apparatus as claimed in claim21 additionally including means operable to maintain said divertor inits normal inoperative position upon the production of the second outputsignal as a consequence of the passage of one or more sheets having anoverall length greater than any length within a predetermined range oflengths.
 23. Apparatus as claimed in claim 1 wherein said means forreceiving and utilizing said output signals includes a first digitalcounter coupled to said first differential amplifier means for receivingsaid first output signal therefrom and to be actuated thereby, and asecond digital counter coupled to said second differential amplifiermeans for receiving said second output signal therefrom and to beactuaTed thereby.
 24. Apparatus as claimed in claim 23 wherein saidfirst digital counter is coupled to said second differential amplifiermeans for inhibiting a first output signal to said first digital counterupon the production of a second output signal.